SUGAR 419 



When hydrolysed by emulsin it yields glucose and saligenin 

 (o Hydroxybenzyl alcohol). The latter is oxidised to salicylic 

 acid, a very important remedy for rheumatic affections, 

 though now made almost exclusively by synthesis from phenol. 



Coniferin is a glucoside which is present in the cambium sap of 

 various fir trees. When hydrolysed by emulsin it yields glucose 

 and coniferyl alcohol, C 6 H 3 (OH)(OCH 3 )-CH : CH-CH 2 OH, which 

 by careful oxidation yields vanillin, C 6 H 3 (OH)(OCH 3 )-CHO, the 

 fragrant constituent of the vanilla pod. 



The production of indigo from the plant has already been 

 described (see Dyes), and it was explained that the blue colouring 

 matter does not occur ready formed in the juice, but is deposited 

 as the result of a kind of fermentation. It is present in the form 

 of indican, a glucoside which splits up on hydrolysis into glucose 

 and indoxyl, from which by oxidation in contact with air indigo- 

 blue is produced. 



Amygdalin, described above, is not the only glucoside which 

 yields prussic acid. Researches within recent years have shown 

 that glucosides which, on hydrolysis by their own enzymes, yield 

 this poisonous product are more widely spread than was formerly 

 supposed. Thus the Lotus arabicus, a small leguminous plant 

 resembling a vetch, which grows abundantly in the valley of 

 the Nile and is used as fodder, contains a yellow crystalline 

 glucoside together with an enzyme, and when moistened with 

 water and crushed the leaves of the plant evolve prussic acid. 

 Hence the plant is very poisonous to cattle, and the effect is 

 most marked in the young plant up to the period of seeding. 

 Some other plants eaten by cattle contain similar substances 

 and require to be used with caution. 



Tannin, which is a very widely diffused vegetable principle 

 which constitutes the astringent agent in many barks, woods, 

 leaves, and other parts of plants, is also a glucoside. It yields 

 gallic acid and glucose when hydrolysed. Tannins are not only 

 protective to the plant, but like other glucosides provide a store 

 of reserve material to be used when the plant requires help in 

 the development of buds or leaves or in the ripening of fruit. 



Starch and other carbohydrates serve a similar purpose. 



It would be unsuitable to these pages to enter into the neces- 

 sarily long story of the successive steps by which, already long 

 ago, the general character and chemical constitution of the 

 sugars have been unfolded. The history of the researches on the 



